In-situ formation of bismuth nanoparticles on nickel foam for ambient ammonia synthesis via electrocatalytic nitrogen reduction

Article abstract of DOI:10.1016/j.jallcom.2021.160006

Bismuth has been regarded as a promising electrocatalyst for triggering nitrogen reduction to ammonia, due to the ease of nitrogen dissociation rendered by the strong interaction between Bi 6p band and the N 2p orbitals. However, the poor conductivity of bismuth limits the electron transfer for nitrogen reduction. In addition, the sluggish water dissociation on the bismuth surface leads to insufficient proton supply for the protonation step of *N₂, causing inferior ammonia production performance. In this work, we prepare an integrated and binder-free bismuth nanoparticles@nickel foam electrode for ambient ammonia synthesis via a facile displacement reaction. Using nickel foam as the conductive substrate improves the electron transfer of bismuth for nitrogen reduction to ammonia. In addition, enhanced water dissociation on the nickel surface improves the protonation of *N₂ by supplying adequate protons via hydrogen spillover, thus boosting the ammonia production performance. This integrated electrode eliminates the use of polymer binders and reduces the contact resistance between the diffusion layer and catalyst layer, facilitating electron delivery and reducing cell resistance, thus requiring less energy input for ammonia production. The performance examination in an electrochemical H-type cell shows that an ammonia yield rate as high as of 9.3 × 10^?11 mol s^?1 cm^?2 and a Faradaic efficiency of 6.3% are achieved. An ammonia yield rate of 8.19 × 10^?11 mol s^?1 cm^?2 is observed after 6 cycles, with a retention rate of 88%.

Full text of DOI:10.1016/j.jallcom.2021.160006

Journal of Alloys and Compounds 875 (2021) 160006
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
In-situ formation of bismuth nanoparticles on nickel foam for ambient
ammonia synthesis via electrocatalytic nitrogen reduction
Guangzhe Li, Zhefei Pan, He Lin, Liang An⁎
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bismuth has been regarded as a promising electrocatalyst for triggering nitrogen reduction to ammonia, due
to the ease of nitrogen dissociation rendered by the strong interaction between Bi 6p band and the N 2p
orbitals. However, the poor conductivity of bismuth limits the electron transfer for nitrogen reduction. In
addition, the sluggish water dissociation on the bismuth surface leads to insufficient proton supply for the
protonation step of *N2, causing inferior ammonia production performance. In this work, we prepare an
integrated and binder-free bismuth nanoparticles@nickel foam electrode for ambient ammonia synthesis
via a facile displacement reaction. Using nickel foam as the conductive substrate improves the electron
transfer of bismuth for nitrogen reduction to ammonia. In addition, enhanced water dissociation on the
nickel surface improves the protonation of *N2 by supplying adequate protons via hydrogen spillover, thus
boosting the ammonia production performance. This integrated electrode eliminates the use of polymer
binders and reduces the contact resistance between the diffusion layer and catalyst layer, facilitating
electron delivery and reducing cell resistance, thus requiring less energy input for ammonia production. The
performance examination in an electrochemical H-type cell shows that an ammonia yield rate as high as of
9.3 × 10−11 mol s−1 cm−2 and a Faradaic efficiency of 6.3% are achieved. An ammonia yield rate of 8.19 × 10−11
mol s−1 cm−2 is observed after 6 cycles, with a retention rate of 88%.
Received 25 December 2020
Received in revised form 6 April 2021
Accepted 15 April 2021
Available online 22 April 2021
Keywords:
Ambient ammonia synthesis
Electrocatalytic nitrogen reduction
Bismuth nanoparticles
Nickel foam
In-situ formation
Binder-free electrode
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
and solar [11], thus contributing to energy and environmental sus-
tainability.
Ammonia is not only a key industrial raw material to produce
fertilizers, textiles and drugs [1,2], but also a promising energy car-
rier due to its high theoretical energy density (3 kW h kg−1) and
hydrogen content (17.7 wt%) [3]. Ammonia, with a global output of
around 150 million tons annually, is primarily produced by the
Haber-Bosch process, which converts N2 and H2 to ammonia by
using a metal catalyst at high temperatures (400–500 °C) and
pressures (20–30 MPa) [4–7]. This process is highly energy intensive,
consuming over 1% global fossil fuels to reach such harsh reaction
condition, and thus leading to 1.6% CO2 emission per year. In re-
sponse to energy and environmental consequences, an alternative
approach to produce ammonia, i.e., electrochemical ammonia
synthesis [8–10], has been recently developed, capable of converting
atmospheric nitrogen and water to ammonia and oxygen (N2+ 3H2O
→ 2NH3+ 3/2O2) under ambient condition. The electrochemical
synthesis method can be driven by intermittent energy such as wind
The first step for realizing electrochemical nitrogen reduction to
ammonia is the dissociation of nitrogen molecules. However, nitrogen
is a stubbornly stable molecule with strong and nonpolarizable triple
bonds, which requires a large overpotential to activate. Hence, an
efficient electrocatalyst is highly desired to reduce the energy barrier
for nitrogen dissociation. Until now, noble metal-based electro-
catalysts [12], such as Au [13–15] and Ru [16–18], are reported as
promising electrocatalysts for ammonia production. However, their
scarcity and high cost hinder their industrialization for large-scale
ammonia production. Alternatively, some non-noble metal-based
electrocatalysts, such as Ni-based materials [19,20], can also be used
for ammonia production. However, Ni-based materials often suffer
from sluggish nitrogen dissociation due to their low N binding energy
for nitrogen activation (* + N2 → *N2), thus lowering the ammonia
production rate. Recently, bismuth is reported as a promising elec-
trocatalyst for ammonia production [21,22], due to the strong inter-
action between the Bi 6p and the N 2p orbitals for nitrogen activation.
However, using single bismuth electrocatalyst leads to sluggish water
dissociation (H2O → H+ + OH-) [23], providing insufficient protons for
the protonation of nitrogen (*N2+ 8H+ + 6e- → * + 2NH4+), thus
⁎
Corresponding author.
E-mail address: liang.an@polyu.edu.hk (L. An).
https://doi.org/10.1016/j.jallcom.2021.160006
0925-8388/© 2021 Elsevier B.V. All rights reserved.

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
lowering ammonia production rate. In addition, bismuth possesses
poor electrical conductivity (1.25 × 106 S/m), which inhibits the
electron transfer for nitrogen reduction [24].
peak was measured by Ultraviolet (UV) adsorption spectra using UV-
Vis double beam spectrophotometer (DB-20, Australia).
Herein, we prepare an integrated and binder-free bismuth na-
noparticles@nickel foam (BiNPs@NF) electrode for ambient am-
monia synthesis via a facile displacement reaction. The fabrication of
BiNPs@NF improves the poor conductivity of bismuth, due to the
high conductivity of nickel (1.43 × 107 S/m). In addition, the higher
electronegativity of bismuth (2.02) over nickel (1.91) also leads to
accumulation of electrons at the side of bismuth, thus improving the
intrinsic activity of bismuth electrocatalysts [25]. Moreover, strong
dissociation of water on nickel surface [26] will generate a great
amount of protons [27] at a high frequency, which will be trans-
ferred to adjacent bismuth surface by a spillover effect [28] via dif-
fusion. The supply of protons can facilitate the nitrogen dissociation
on the bismuth surface by protonation of *N2, forming NH3 (*N2+
8H+ + 6e- → * + 2NH4+). In addition, the fabrication of BiNPs@NF
electrode eliminates the use of polymer binders, which would create
a barrier for electron delivery. Compared to the conventional elec-
trode containing a diffusion layer and a catalyst layer and thus
causing a large contact resistance between two layers [29], this in-
tegrated electrode reduces the contact resistance, thus requiring less
energy input for ammonia production. By performance evaluation in
an H-type cell, an ammonia yield rate of 9.3 × 10−11 mol s−1 cm−2 is
achieved and a high Faradaic efficiency of 6.3% is observed. In ad-
dition, an ammonia yield rate of 8.19 × 10−11 mol s−1 cm−2 is found
after 6 cycles with a retention rate of 88%.
2.3. Electrochemical measurements
A three-electrode electrochemical system was constructed for
the examination of nitrogen reduction reaction (NRR) properties of
electrocatalysts. Typically, BiNPs@NF (1.0 × 2.0 cm2) was used as the
working electrode, commercial graphite plate (Hebei thermal in-
sulation engineering Co., Ltd) with a geometric surface area of
2.0 cm2 was used as the counter electrode and Hg/Hg2SO4 (MSE) was
used as the reference electrode. The mass loading of BiNPs@NF was
around 1.28 mg cm−2. All the potentials were converted to reversible
hydrogen electrode (RHE) [30]:
ERHE = EMSE + 0.059 pH + 0.616
(1)
0.5 M K2SO4 solution was used as the electrolyte and Nafion 115
membrane was also employed to separate cathode and anode
compartments and transport ions during cell operation. The volume
of electrolyte in each chamber was 100.0 mL. The cathodic reaction
is the NRR [31]:
–
+
N2 + 8H+ + 6e → 2NH4 Ec = −0.078 pH + 0.274 V vs.RHE
The anodic reaction is the oxygen evolution reaction [32]:
3H2O → 6H+ + 6e- + O2 Ea = 1.23 V vs.RHE
(2)
(3)
Therefore, the overall reaction in neutral aqueous media can be
expressed:
2. Experimental sections
+
N2 + 2H+ + 3H2O → 2NH4 + 3/2O2 Eo = 0.956 + 0.019 pH
(4)
2.1. Pretreatment and preparation
At the inlet of cathode compartment, an acid trap containing
0.05 M H2SO4 was used to remove the ammonia contamination from
the nitrogen source. In addition, an alkaline trap containing 0.05 M
NaOH was used to remove the NOx contamination from the nitrogen
source. At the outlet, an acid trap containing 0.05 M H2SO4 and a
liquid seal was used to avoid the contamination from the air. All
electrochemical measurements were carried out using Multi-
Autolab electrochemical workstation (M204, Switzerland). Before
cell operation, N2 gas was bubbled into the cathode compartment for
1 h to exhaust the residual air in the chamber. A gas flowmeter was
used to keep the gas flow rate at 20 mL min−1 during cell operation.
In addition, a magnetic stirrer was used with a stirring rate of 200 r
min−1 to help dissolution and even dispersion of N2 gas. For the
cyclic test, the electrocatalytic electrode previously used was re-
peatedly washed with DI water and then dried. The comparison test
using Ar gas followed the same procedure as the N2 test.
BiNPs@NF was prepared via a simple displacement reaction. A
strip of nickel foam (1.0 × 2.0 cm2) was first washed by DI water and
ethanol for several times. To remove the oxide layer of nickel foam, it
was annealed in the mixture gas of H2 (5.0 vol%) and Ar (95.0 vol%)
for 2 h at 800 °C. Next, it was immersed in the anhydrous glycol
containing 0.1 mM Bi(NO3)3.5H2O in a glove box filled with Ar gas.
For the preparation of BiNPs, the nickel foam was kept in anhydrous
glycol at room temperature for 18 h. For the preparation of bismuth
nanoclusters (BiNCs), the nickel foam was kept in anhydrous glycol
solution at 40 °C for 18 h. For the preparation of bismuth microplates
(BiMPs), it was kept in anhydrous glycol solution at an elevated
temperature of 80 °C for 18 h. After displacement reaction, the
BiNPs@NF was taken out and washed by DI water and ethanol again.
Subsequently, the BiNPs@NF was kept in the glove box filled with Ar
gas to avoid surface oxidation of Bi. It was found that the surface of
BiNPs will be oxidized and become Bi2O3 if it was kept in air for 2–3
days. For the pretreatment of Nafion membrane, it was boiled in
1.0 M NaOH solution for 2 h and subsequently boiled in DI water
for 2 h.
2.4. Detection and quantification of ammonia and hydrazine
The detection and quantification of ammonia ions were achieved
by indophenol blue method [33]. First, chromogenic reagent was
prepared by dissolving sodium citrate (3.0 wt%) and salicylic acid
(3.0 wt%) in 1.0 M NaOH solution. For the color rendering of the test
solution, 1.0 mL chromogenic solution was mixed with 0.5 mL so-
dium hypochlorite solution (5.0 wt%) and 0.1 mL sodium ni-
troferricyanide dihydrate (1.0 wt%), followed by adding 1.0 mL test
solution. After 2 h, the mixed solution was transferred to a cuvette
for the adsorption test, by measuring the adsorption peak intensity
at 651 nm. For the measurement of standard adsorption curves of
ammonia solution with known concentrations, a series of NH4Cl
solutions with different concentrations (0.0625, 0.125, 0.25, 0.5, 1.0
and 2.0 mg L−1) containing 0.5 M K2SO4 were first prepared. Then,
2.2. Characterizations
The phase analysis of BiNPs@NF was conducted by X-ray dif-
fraction (XRD) (D/max 2500PC) and the element analysis was con-
ducted by energy-dispersive X-ray (EDX). The surface morphology of
BiNPs@NF was observed by scanning electron microscopy (SEM)
(Tescan MAIA3, Japan). The valence states of elements were analyzed
by X-ray photoelectron spectroscopy (XPS), recorded by a spectro-
meter with Mg/Al Kα radiation (Thermo VG Scientific, USA). As for
the detection of ammonia ions in the test solution, the absorbance
2

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
Fig. 1. (a) Thermodynamic feasibility of in-situ formation of bismuth on nickel foam at room temperature. (b) XRD pattern showing the successful loading of bismuth on nickel
foam. XPS spectra of (c) bismuth confirming the existence of Bi0 and (d) Ni 2p peaks of BiNPs@NF electrode and bare nickel foam without electrocatalysts.
1.0 mL standard NH4Cl solution was mixed with the coloring reagent
following the same procedure as the measurement of the test so-
lution. Resultantly, the correlation between absorbance and am-
monia concentration was established (y = 0.25954 x - 0.00584) and a
good linear fit (R2 = 0.9996) was observed.
3. Results and discussion
The formation of bismuth on nickel via displacement reaction is
thermodynamically feasible, as shown in Fig. 1a, according to:
2Bi3+ + 3Ni → 3Ni2+ + 2Bi ΔGθ = −17.94 kJ mol−1
(5)
The detection of hydrazine in the test solution was achieved via
the Watt and Chrisp method [34]. First, the color reagent was pre-
pared by dissolving 0.199 g C9H11NO into a mixture of 1.0 mL con-
centrated HCl (38 wt%) and 10.0 mL absolute ethanol. Then, 1.0 mL
color reagent was mixed with 1.0 mL test solution for color ren-
dering. After 20 min, it was transferred to UV–vis double beam
spectrophotometer, and the intensity of the absorption peak was
measured at 457 nm. For the measurement of standard adsorption
curves of hydrazine solution with known concentrations, a series of
hydrazine solutions with different concentrations (0.15625, 0.3125,
0.625, 1.25 and 2.5 mg L−1) containing 0.5 M K2SO4 were first pre-
pared, followed by mixing with color reagents for adsorption tests.
As a result, a relation between hydrazine concentration and absor-
bance was established (y=0.05588+ 0.96238 x) with a good linear fit
(R2 = 0.9996).
Therefore, we attempt to prepare BiNPs@NF by immersing nickel
foam into the anhydrous ethylene glycol containing 0.1 mM Bi
(NO3)3. BiNPs@NF can be prepared by immersing nickel foam in the
ethylene glycol solution for 18 h at room temperature, as revealed by
the XRD pattern (Fig. 1b). Three characteristic diffraction peaks at
27.16°, 37.95° and 39.62° are indexed to the (012), (104) and (110)
planes of bismuth (JCPDS no. 44-1246), respectively [35]. XPS
spectra shows that the concentration (wt%) of Bi and Ni on the
surface of BiNPs@NF electrode is 64.59% and 35.41%, respectively,
containing no other impurities. The valence state of Bi0 is confirmed
by observing two separated peaks centered at 157.08 eV and
162.48 eV, which can be identified as Bi 4f7/2 and Bi 4f5/2 signals of
Bi0 [35], respectively, as shown in Fig. 1c. In addition, Ni 2p peaks
centered at 855.68 eV and 873.18 eV are also observed (Fig. 1d),
3

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
Fig. 2. (a-b) Surface morphologies of BiNPs@NF and its phase analysis by (c-e) elemental mapping and (f) EDX pattern.
corresponding to Ni2p3/2 and Ni2p1/2, respectively. We found that
bismuth is deposited on the nickel foam surface without forming the
Bi/Ni alloy, as revealed by XPS spectra (Fig. 1d). The peak of Ni 2p will
be shifted to a higher binding energy if Bi/Ni alloy forms [25].
However, the as-prepared bismuth nanoparticles@nickel foam dis-
plays no peak shift of Ni 2p, as compared to original nickel foam.
BiNPs are evenly distributed on the entire nickel foam skeleton
(Fig. 2a-b), with an average particle size of 60 nm, as shown in the
inset figure of Fig. 2a. The successful growth and even distribution of
BiNPs are further demonstrated by elemental mapping (Fig. 2c-e)
and EDX pattern (Fig. 2f). We also found that by elevating the re-
action temperature, the surface morphologies of bismuth can be
significantly changed. Elevating the temperature to 40 °C leads to the
formation of BiNCs, with a thickness of around 100 nm, as shown in
Fig. S1. Such a significant change can be attributed to the accelerated
reaction kinetics at an elevated temperature. When further elevating
the temperature to 80 °C, BiMPs are formed and these microplates
are stacked upwards, protruding away from the nickel skeleton
surface (Fig. S2). The size of each plate is around tens of micrometers
and the thickness of each plate is about 2 micrometers. The EDX
pattern (Fig. S3) of BiMPs shows distinct Bi signals due to their large
particle size, in sharp contrast to that of BiNPs with weak Bi signals
(Fig. 2f).
different conditions of fed gases (N2 or Ar). When using bare nickel
foam as the working electrode (Fig. 3a), there is no current difference
under different conditions of fed gases, showing that bare nickel
foam cannot be used for nitrogen reduction. The onset potential for
electrochemical reaction is − 0.18 V (vs. RHE), which is attributed to
the hydrogen evolution reaction (HER) [36–38]. However, when
using BiNPs@NF as the working electrode, the onset potential is
negatively shifted to − 0.27 V. The possible reason for this is that
BiNPs can suppress the hydrogen evolution [39], since the current
density for HER is significantly reduced by around 50% (Fig. 3b). A
current gap is observed when using BiNPs@NF as working electrode
with different fed gases (Fig. 3a). Since Ar is a highly inert gas which
cannot be electrochemically reduced, the additional current can only
be contributed by the reduction of N2 gas, thus revealing a voltage
window for NRR from − 0.375 V to − 0.65 V. The current density for
NRR (jNRR) can be obtained by calculating the current difference of
LSV curves, as shown in Fig. 3c. A maximum jNRR is observed at
− 0.525 V, and the decreased jNRR observed at a more negative po-
tential is because of the enhanced HER [40]. The Faradaic efficiency
(FE) for NRR can also be obtained by the following equation [22]:
FENRR (%) = (jN
jAr )/jN
2
(6)
2
A maximum FE of 6.8% for NRR is observed at − 0.475 V, as shown
in Fig. 3d. The NRR properties of BiNCs and BiMPs electrocatalysts
are also investigated by LSV tests (Figs. S5 and S6). Similarly, the
appearance of current gap also demonstrates the capability of NRR
when using BiNCs@NF and BiMPs@NF as the working electrode. It is
found that BiNPs@NF offers the best NRR property among three
electrodes (Fig. S7), indicating that the size of electrocatalysts plays a
decisive role in the NRR property. A smaller size of bismuth crystals
can expose more active sites for NRR.
NRR active sites are the three-phase reaction interface, including
nitrogen (gas), protons (liquid) and electrons (solid). Their role is
capable of simultaneously accepting nitrogen, protons and electrons
for electrochemical nitrogen reduction (N2+ 8H+ + 6e- → 2NH4+). The
NRR properties of BiNPs@NF are investigated using a setup con-
taining acid trap, gas flowmeter, magnetic stirrer and a three-elec-
trode electrochemical cell (Fig. S4). The voltage window for NRR is
examined by linear sweep voltammetry (LSV) tests operating under
4

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
Fig. 3. (a) LSV tests under various operating conditions. (b) HER comparison between BiNPs@NF and nickel foam. (c) Total current and current distribution for NRR and HER. (d)
Faradaic efficiencies.
Theoretically, there are two possible products of electrochemical
NRR, i.e., ammonia ions and hydrazine [31]. From a thermodynamics
perspective, it is possible to generate both kinds of products when
operating the cell at a potential below − 0.332 V (vs. RHE), according
to the Eq. 2 and Eq. 7 [31]:
concentration in the cathode chamber after cell operation can be
determined by comparing with the standard UV adsorption curves
(Fig. S10). Hence, the ammonia yield rate can be obtained [41]:
rNH3 = (n × V)/(t × A)
(8)
Where n is the ammonia concentration, V represents the volume of
the electrolyte, t stands for the reaction time and A is the geometric
area. The Faradaic efficiency for ammonia production can be calcu-
lated as follow [41]:
–
N2 + 4H2O + 4e → N2H4 + 4OH- Eo = −0·332 V vs·RHE
(7)
For the examination of ammonia and hydrazine production, cell
is operated at different potentials from − 0.4 V to − 0.6 V for 2 h
(Fig. 4a). The solution in cathode chamber is collected after cell
operation and mixed with color reagent for UV–vis adsorption. As
for the detection of ammonia ions, a highest adsorption peak is
observed at − 0.5 V centered at 652 nm, as shown in Fig. 4b, in-
dicating the maximum amount of ammonia production is achieved
at − 0.5 V. In comparison, no adsorption peak is observed under Ar
condition (Fig. S8). This demonstrates that the ammonia production
only results from the NRR, eliminating the possibility of ammonia
contamination during cell operation. In addition, no adsorption peak
appears when using nickel foam as working electrode (Fig. S8), in-
dicating there is no contribution from the nickel foam substrate for
ammonia production. Moreover, when operating the cell under
open-circuit condition for 2 h in Ar, there is no ammonia production,
confirming the stability of working electrode (Fig. S9). The ammonia
FE (%) = (3F × n × V)/(M × Q)
(9)
Where F is the Faraday constant, M represents the relative molecular
mass of NH3, and Q stands for the total consumed charge during cell
operation. The ammonia production performance is summarized in
Fig. 4c, showing a maximum ammonia yield rate of 9.3 × 10−11
mol s−1 cm−2 (or 5.68 μg h−1 cm−2) at − 0.5 V (vs. RHE), which is
comparable with recently reported electrocatalysts (Table S1). Si-
multaneously, a Faradaic efficiency of 6.3% is observed. As for the
cyclic test of BiNPs@NF electrode, a good ammonia yield rate re-
tention of 88% is observed after 6 cycles, showing that BiNPs@NF
electrode can be reversibly used for ammonia production. The high
ammonia production rate results from the synergistic effect between
bismuth and nickel (Fig. 5). Using single bismuth electrocatalyst
leads to sluggish protonation step due to the insufficient proton
5

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
Fig. 4. (a) Electrocatalytic NRR at different potentials. (b) UV adsorption spectra after 2 h cell operation. (c) Ammonia production performance. (d) Cyclic stability.
supply caused by poor catalytic properties of water dissociation of
bismuth [23], as shown in Fig. 5a. Using single nickel electrocatalyst
also results in poor ammonia production performance due to its
incapability of nitrogen adsorption (Fig. 5b). In comparison, using
BiNPs@NF electrode facilitates the nitrogen adsorption and si-
multaneously improves the proton supply by hydrogen spillover
effect on nickel foam [28], thus facilitating the nitrogen reduction to
ammonia (Fig. 5c). As for the hydrazine production, the solution in
cathode chamber is collected and mixed with coloring reagent for
the adsorption test. Resultantly, no adsorption peak appears when
operating the cell at different potentials (Figs S11 and S12), sug-
gesting no hydrazine production during the NRR.
4. Concluding remarks
In summary, we prepare an integrated and binder-free bismuth
nanoparticles@nickel foam electrode via a facile displacement re-
action for efficient electrochemcial ammonia synthesis. The sy-
nergistic effect between bismuth and nickel facilitates nitrogen
reduction to ammonia, in which bismuth acts as active sites for ni-
trogen adsorption (* + N2 → *N2) and nickel with strong water dis-
sociation can supply sufficient protons for protonation of *N2 (*N2+
8H+ + 6e- → * + 2NH4+). This integrated electrode also eliminates the
use of binders and reduces the contact resistance between diffusion
Fig. 5. A comparison of catalytic properties of different electrodes. (a) The deposition
of bismuth on inactive conductive support. (b) Nickel foam. (c) The deposition of
bismuth nanoparticles on nickel foam.
layer and catalyst layer, promoting the electron delivery for nitrogen
reduction to ammonia. The performance examination in an
6

G. Li, Z. Pan, H. Lin et al.
Journal of Alloys and Compounds 875 (2021) 160006
electrochemical H-type cell displays a high ammonia yield rate of
9.3 × 10−11 mol s−1 cm−2 and a Faradaic efficiency of 6.3%. This sig-
nificant advance opens a window of opportunity for breakthroughs
in the development of ambient ammonia synthesis technologies.
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CRediT authorship contribution statement
L.A. conceived the idea and supervised the project. G.L. and Z.P.
designed the experiments. G.L. performed the experiments and
prepared the manuscript. All of the authors analyzed the data and
commented on the manuscript.
Correspondence and requests for materials should be addressed
to L.A.
Declaration of Competing Interest
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The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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